In operating such a product treatment apparatus, great care has to be exercised so as to ensure that the tension applied to the treated product emerging from the treatment means does not increase to a level at which the tension consequently induced in the extrusion product as it emerges from the extrusion apparatus is sufficient to break or otherwise impair the properties of the extrusion product entering the treatment means. Control difficulties can arise since, in particular, the yield stress of the hot extrusion product is variable in dependence upon the temperature at which the extrusion product emerges from the extrusion apparatus, which temperature is itself dependent upon the rate at which the extrusion product issues from the extrusion apparatus, and the general operating temperature of the extrusion apparatus.